Pulse oximetry is at present the standard of care for continuous monitoring of arterial oxygen saturation (SpO2). Pulse oximeters provide instantaneous in-vivo measurements of arterial oxygenation, and thereby an early warning of arterial hypoxemia, for example.
A pulse oximeter comprises a computerized measuring unit and a sensor attached to the patient, typically to a finger or ear lobe. The sensor includes a light source for sending an optical signal through the tissue and a photo detector for receiving the signal after transmission through the tissue. On the basis of the transmitted and received signals, light absorption by the tissue can be determined. During each cardiac cycle, light absorption by the tissue varies cyclically. During the diastolic phase, absorption is caused by venous blood, tissue, bone, and pigments, whereas during the systolic phase there is an increase in absorption, which is caused by the influx of arterial blood into the tissue. Pulse oximeters focus the measurement on this arterial blood portion by determining the difference between the peak absorption during the systolic phase and the constant absorption during the diastolic phase. Pulse oximetry is thus based on the assumption that the pulsatile component of the absorption is due to arterial blood.
Light transmission through an ideal absorbing sample is determined by the known Lambert-Beer equation as follows:Iout=Iine−εDC  ,(1)
where Iin is the light intensity entering the sample, Iout is the light intensity received from the sample, D is the path length through the sample, ε is the extinction coefficient of the analyte in the sample at a specific wavelength, and C is the concentration of the analyte. When Iin, D, and ε are known, and Iout is measured, the concentration C can be calculated.
In pulse oximetry, in order to distinguish between two species of hemoglobin, oxyhemoglobin (HbO2), and deoxyhemoglobin (RHb), absorption must be measured at two different wavelengths, i.e. the sensor normally includes two different light emitting diodes (LEDs). The wavelength values widely used are 660 nm (red) and 940 nm (infrared), since the said two species of hemoglobin have substantially different absorption values at these wavelengths. Each LED is illuminated in turn at a frequency which is typically several hundred Hz.
Conventional pulse oximeters are restricted to measurement of arterial oxygen saturation at a single tissue site. Therefore, if continuous and simultaneous oxygen status measurements are needed from several tissue sites, a straightforward method is to use a plurality of conventional pulse oximeters simultaneously. The need may arise, for example, during a delivery when both the mother and the infant need to be monitored simultaneously.
To eliminate the above drawback, pulse oximeters have been developed, which provide simultaneous and continuous measurement results from a plurality of tissue sites. U.S. Pat. No. 6,714,804 discloses a stereo pulse oximeter providing simultaneous and continuous oxygen status measurements at multiple tissue sites. The pulse oximeter is provided with multiple sensors attachable to distinct tissue sites. Each sensor may be connected through a separate cable and sensor interface to a signal processor. Alternatively, a so-called stereo sensor, which is provided with multiple branches, may connect the sensors through a common patient cable to a single connection at the pulse oximeter. From the said single connection each sensor signal is branched off to the respective sensor interface.
U.S. Pat. No. 5,218,962 further discloses a multiple region pulse oximetry probe and oximeter, which enable the blood characteristics to be sensed at two or more unique sites. In one embodiment, the probe housing accommodates probe elements for two distinct tissue regions, but the probe elements may also be mounted in separate probe housings. The oxygen saturation values obtained from two tissue sites are compared with each other to improve the reliability of the measurement.
A major problem in pulse oximetry concerns mechanical or motion artefacts, i.e. unwanted motion of the patient, which causes extra noise in the signal and may thus result in inaccurate SpO2 readings and false alarms. To fight the motion artefact, pulse oximeters may be provided with motion sensors which detect patient movements. U.S. Pat. No. 5,025,791, for example, discloses a pulse oximeter with a physical motion sensor. If the motion sensor detects motion that continuously lasts longer than for a predetermined time, the photoelectric measurement signals are not utilized or the obtained readings are provided with an indication of the simultaneous physical motion.
A drawback related to current pulse oximeters providing simultaneous measurement results from a plurality of sensors is the rather extensive multiplication of the hardware required by the parallel measurements. As mentioned above, each sensor normally requires a dedicated interface that typically includes both signal processing means for the electric signal received from the respective sensor and current drivers for the emitters of the respective sensor. In a pulse oximeter provided with a motion sensor the integration of the motion sensor into the same device tends to convert the pulse oximeter to a dedicated device for which standard pulse oximeter components cannot anymore be utilized. The integration thus considerably increases the complexity of the device.
The present invention seeks to eliminate the above drawbacks and to bring about a novel mechanism for non-invasively obtaining measurement signals from a patient through a plurality of optical sensors.